Conventionally a measuring microscope or an image measuring apparatus is widely spread as a measuring apparatus which measures a shape size of a workpiece of the specimen. In such measuring apparatuses, the specimen which is placed on a horizontally movable stage is enlarged and projected to an image picking-up apparatus through an optical system, and a surface shape, defect inspection, a coordinate, and dimensions of the specimen are measured by utilizing an image signal into which the projection image is converted. For example, Japanese Patent Publication Laid-Open No. 11-183124 (Patent Document 1) discloses such a measuring apparatus.
Usually the measuring apparatus includes a transmission illumination and an epi-illumination which are of an illumination system, and the epi-illumination is exclusively used when an opaque object such as a machine component is measured. The epi-illumination includes a halogen lamp, a collector lens, and a condenser lens, and the object is illuminated in a telecentric manner through an objective lens.
On the other hand, an image-formation optical system includes an objective lens and an image-formation lens, and an image of the specimen is formed on the image picking-up. There is well known an apparatus including a zoom optical system which can vary magnification according to dimensions of the specimen.
The illumination light in which a balance is established between a wide visual field on a zoom low-power side and a high numerical aperture (NA) on a zoom high-power side is required in the illumination of the apparatus including the zoom optical system. However, in the conventional epi-illumination system, unfortunately NA on the zoom high-power side may be insufficient or a neighborhood of the visual field becomes dark on the zoom low-power side. Recently, with wide spread use of light emitting diode (LED), LED is increasingly adopted as a light source of the illumination optical system. LED has various merits such as lower power consumption, a constant color temperature, and rapid responsibility compared with halogen lamps. However, because the light-emitting area is smaller than that of the halogen lamps, the problems mentioned above become more evident.
In view of the foregoing, an object of the invention is to provide an image measuring apparatus including a zoom optical system, the image measuring apparatus in which lack of NA and uneven visual field in the range of zoom low power to the zoom high power can be prevented using the epi-illumination in which a small-area light source such as LED is used.